This invention relates to apparatuses and methods for deploying logging tools and signalling in boreholes.
The logging of boreholes hitherto has used techniques that are well known in the oil and gas industries. The advantages of such an activity are known to those skilled in the art of oil and gas production.
When a borehole is drilled, it is seldom smooth and regular. Sections of the borehole sometimes cave in. Sometimes there are other sections of rock, in particular shales and clays, that squeeze into the borehole as a result of pressure exerted by overlying strata.
Traditionally, borehole logging has involved the use of a so-called wireline logging tool. The wireline logging tool is lowered on a wireline or pushed on drillpipe into the borehole to a downhole, logging location. The wireline logging tool is connected by a wireline to eg. data processing and recording apparatus at a surface location external of the borehole.
Wireline logging tools are of comparatively large diameter. Consequently it is difficult to push or lower a wireline logging tool into a borehole having caved in or squeezed sections as aforesaid.
In recent years it has become known to employ, for the logging of boreholes, a so-called compact logging tool comprising logging tool sections and battery/memory sections. This logging tool typically is of considerably less diameter than a conventional wireline logging tool. It includes a self-contained power supply in the form of a series of batteries; and one or more memory devices, whose function is to record data logged by the logging tool.
Battery/memory logging tools in many circumstances offer advantages over traditional, wireline tools.
It is now known to deploy such a battery/memory logging tool using a so-called “garaging” technique, in which the tool lies retracted within one or more joints of drillpipe during running in of the drillpipe at tripping speed. Once the drillpipe reaches the total depth (“TD”) of the well, a mechanism is actuated to cause delatching of a delatchable running sub that during running in of the drillpipe causes retention of the battery/memory logging tool within the drillpipe.
Delatching of the running sub causes deployment of the logging tool to a location protruding from the downhole end of the drillpipe, at which location the logging tool is available for logging operations. Such operations then occur as the drillpipe is withdrawn upwardly from the wellbore. The battery/memory logging tool logs data on the open hole well as it travels upwards towards an uphole location, supported on the end of the drillpipe.
Following withdrawal of all the joints of drillpipe in the wellbore, the memory section of the battery/memory logging tool is recovered. The data recorded therein is downloaded, enhanced and/or analysed as desired.
The known technique for deploying the logging tool includes circulating the well with fluid under pressure, by means of a positive displacement pump connected to the drillpipe at an uphole (surface) location.
This permits the insertion into the drillpipe of a messenger sub. Such a sub is pumpable within the drillpipe to the downhole end thereof, where it operates a release tool. Operation of the release tool causes delatching of the running sub and deployment of the logging tool as aforesaid.
The above-described method has proved highly successful in the data logging art.
Nonetheless there is a need for further improvements in the efficiency of deployment of logging tools.
Rig time is costed at several hundred or thousand dollars per hour. Therefore it is strongly desirable to complete data logging operations in as short a time as possible. However the time taken to pump the messenger sub from an uphole location to approximately the TD of the well can be significant, not least because most oil wells are many hundreds or thousands of meters long.
The drillpipe must be of the correct diameter, and drifted, to ensure that the messenger will pass through the drillpipe and any bottom hole restrictions. Such preparation of the drillpipe is also time-consuming.
The known garaging technique for the deployment of logging tools includes method steps aimed at signalling from a downhole location to an uphole location whether deployment of the logging tool has commenced. There is however a greater need for communication between downhole and uphole locations in oil wells as the logging tools become more complex.
There have been numerous proposals in the past aimed at providing such communication without resorting to wireline connections between the downhole and uphole ends of a drillpipe. In the main such prior art proposals attempt to provide encoded communication between the downhole and uphole locations, by means of acoustic signals generated as pressure pulses in the fluid circulating in the well.
The approach in the prior art has been to develop a language using which it is possible digitally to transmit packets of data within the drillpipe.
This approach suffers from several disadvantages.
Principal among these is the use in the prior art of electromechanical pulsing techniques to generate fluid pressure signals at downhole locations. Typically such techniques involve the use of electrically actuated, mechanical valving members to interrupt the flow of mud (or other fluid) thereby creating pulses of sufficient amplitude to be detectable at a surface location. Since the mass of mud typically requiring to be arrested by the valving members is several thousand kilogrammes the service lives and general reliability of the prior art devices are poor.
A further disadvantage of the prior art techniques is that the speed of data transmission is poor, because of the limited bandwidth of the transmission medium (mud). This problem is acute when attempting to multiplex data transmissions.